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So, I think what you're asking is how to take spatial data from json and join it with some csv data that is loaded separately?

I did something similar with a choropleth map I was drawing and basically I just created a map of topology element ids to data objects and then I did a lookup using the topology element id to get whatever I wanted to associate with the actual drawn map element (I was using this method to set the color for the choropleth based on the fips country code).

So basically, draw the map so that you have an id associated with each map element that you want to be able to hover over. Then, in your mouseover/mouseout handlers, you will use that id to lookup the data you want to show in the tooltip and either use the svg title element or tipsy or manually draw an svg text element or whatever to show the tooltip.

The client could change my data, so I kept it separate and redrew the map each time the data changed so that the colors would update. Since my data was topojson, I could access the feature id from the map data using d.properties.code (because I had joined the codes into the topojson file using the topojson tool I reference above... but you could use whatever unique id is in the spatial data file you have).